Shell: Start implementing a POSIX-compliant parser

The parser is still very much a work-in-progress, but it can currently
parse most of the basic bits, the only *completely* unimplemented things
in the parser are:
- heredocs (io_here)
- alias expansion
- arithmetic expansion

There are a whole suite of bugs, and syntax highlighting is unreliable
at best.
For now, this is not attached anywhere, a future commit will enable it
for /bin/sh or a `Shell --posix` invocation.

Userland/Shell/PosixParser.h

@@ -0,0 +1,117 @@
+/*
+ * Copyright (c) 2022, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <Shell/AST.h>
+#include <Shell/PosixLexer.h>
+
+namespace Shell::Posix {
+
+class Parser {
+public:
+    Parser(StringView input, bool interactive = false)
+        : m_lexer(input)
+        , m_in_interactive_mode(interactive)
+        , m_eof_token(Token::eof())
+    {
+        fill_token_buffer();
+    }
+
+    RefPtr<AST::Node> parse();
+    RefPtr<AST::Node> parse_word_list();
+
+    struct Error {
+        DeprecatedString message;
+        Optional<AST::Position> position;
+    };
+    auto& errors() const { return m_errors; }
+
+private:
+    Optional<Token> next_expanded_token();
+    Vector<Token> perform_expansions(Vector<Token> tokens);
+    void fill_token_buffer();
+
+    Token const& peek() const
+    {
+        if (eof())
+            return m_eof_token;
+        return m_token_buffer[m_token_index];
+    }
+    Token const& consume()
+    {
+        if (eof())
+            return m_eof_token;
+        return m_token_buffer[m_token_index++];
+    }
+    void skip()
+    {
+        if (eof())
+            return;
+        m_token_index++;
+    }
+    bool eof() const
+    {
+        return m_token_index == m_token_buffer.size() || m_token_buffer[m_token_index].type == Token::Type::Eof;
+    }
+
+    struct CaseItemsResult {
+        Vector<AST::Position> pipe_positions;
+        NonnullRefPtrVector<AST::Node> nodes;
+    };
+
+    RefPtr<AST::Node> parse_complete_command();
+    RefPtr<AST::Node> parse_list();
+    RefPtr<AST::Node> parse_and_or();
+    RefPtr<AST::Node> parse_pipeline();
+    RefPtr<AST::Node> parse_pipe_sequence();
+    RefPtr<AST::Node> parse_command();
+    RefPtr<AST::Node> parse_compound_command();
+    RefPtr<AST::Node> parse_subshell();
+    RefPtr<AST::Node> parse_compound_list();
+    RefPtr<AST::Node> parse_term();
+    RefPtr<AST::Node> parse_for_clause();
+    RefPtr<AST::Node> parse_case_clause();
+    CaseItemsResult parse_case_list();
+    RefPtr<AST::Node> parse_if_clause();
+    RefPtr<AST::Node> parse_while_clause();
+    RefPtr<AST::Node> parse_until_clause();
+    RefPtr<AST::Node> parse_function_definition();
+    RefPtr<AST::Node> parse_function_body();
+    RefPtr<AST::Node> parse_brace_group();
+    RefPtr<AST::Node> parse_do_group();
+    RefPtr<AST::Node> parse_simple_command();
+    RefPtr<AST::Node> parse_prefix();
+    RefPtr<AST::Node> parse_suffix();
+    RefPtr<AST::Node> parse_io_redirect();
+    RefPtr<AST::Node> parse_redirect_list();
+    RefPtr<AST::Node> parse_io_file(AST::Position, Optional<int> fd);
+    RefPtr<AST::Node> parse_io_here(AST::Position, Optional<int> fd);
+    RefPtr<AST::Node> parse_word();
+
+    template<typename... Ts>
+    void error(Token const& token, CheckedFormatString<Ts...> fmt, Ts&&... args)
+    {
+        m_errors.append(Error {
+            DeprecatedString::formatted(fmt.view(), forward<Ts>(args)...),
+            token.position,
+        });
+    }
+
+    Lexer m_lexer;
+    bool m_in_interactive_mode { false };
+    Vector<Token, 2> m_token_buffer;
+    size_t m_token_index { 0 };
+    Vector<Token> m_previous_token_buffer;
+
+    Vector<Error> m_errors;
+
+    Token m_eof_token;
+
+    bool m_disallow_command_prefix { true };
+};
+
+}